1. Related Prior Art
The art of producing fluid fertilizers, both solutions and suspensions, is well known and fully described in the literature. See, for example, the following: U.S. Pat. No. 3,697,247, Jones et al, Oct. 10, 1972; U.S. Pat. No. 3,382,059, Getsinger, May 7, 1968; U.S. Pat. No. 3,192,013, Young, June 29, 1965; U.S. Pat. No. 3,813,233, Kendrick, May 28, 1974; U.S. Pat. No. 4,066,432, Jones, Jan. 3, 1978; U.S. Pat. No. 3,109,729, Slack et al, Nov. 5, 1963; U.S. Pat. No. 3,234,004, Smith et al, Feb. 8, 1966; Defensive Publications No. T940,014, Nichols et al, Nov. 4, 1975; No. T911,008, Getsinger, June 5, 1973; Canadian Pat. No. 811,080, Ramaradhya, Apr. 22, 1969; German Offen. No. 2,655,571, MacKinnon, June 20, 1977; British Pat. No. 1,301,005, Standard Oil Company, Dec. 29, 1972; No. 823,449, Brownlie, Nov. 11, 1959.
2. Field of the Invention
High-quality and highly concentrated fluid fertilizers of various types are now in wide use throughout the country because they display certain distinct advantages over dry mixes. The advantages of fluid fertilizers are lower shipping and handling costs, more simplified and even distribution to the soil, and the convenience of incorporation of pesticides, herbicides, and micronutrients in fluid fertilizers. Fluid fertilizers may be either solutions, slurries, or suspensions. Both slurries and suspensions contain crystals of fertilizer salts in saturated solutions. However, suspensions also contain small amounts of a suspending agent which keeps the liquid and solid phases homogeneously distributed. Slurry fertilizers generally have been replaced by suspensions because of the far superior storage and handling properties exhibited by suspensions, as will be discussed later. Suspension fertilizers which contain an abundance of small crystals of fertilizer salts suspended in saturated solutions can be of significantly higher concentration than solution fertilizers which contain no solids. Thus, suspension fertilizers have a distinct economic advantage over solution fertilizers in that the costs of handling, freight, storage, and application are cheaper per unit of plant nutrient. In addition, herbicides, pesticides, and micronutrients can be incorporated in suspensions in solid forms, regardless of their solubility therein, whereas in solution fertilizers, only soluble materials can be incorporated.
3. Description of the More Pertinent Prior Art
Ammonium polyphosphate suspension fertilizers are well known in the industry and have been produced as taught by Getsinger '059, Young '013, Jones et al '247, and Slack et al '729, supra. Because of the relatively high cost of polyphosphates, suspensions containing orthophosphate instead of polyphosphate were developed and prepared as taught by Kendrick '233 and Jones '432, supra.
Nitrogen solution fertilizers of various types are in wide use today. To provide an adequate supply of these types of fertilizers, the utilization of urea in combination with ammonium nitrate to form nitrogen solutions is widespread because higher nitrogen concentrations can be attained than by using solutions of either of these nitrogen sources separately. A common example of this practice is urea-ammonium nitrate solution (32 percent N) produced by mixing together about 35 parts of urea, 45 parts of ammonium nitrate, and 20 parts of water. This solution salts out at about 32.degree. F.; increasing the water content lowers the salting-out temperature, but increases the cost of handling, shipping, storage, and application per unit weight of nitrogen.
Urea-ammonium nitrate suspensions have been produced, as in Nichols et al '014, supra, by dispersing attapulgite clay in water with a dispersing agent, tetrasodium pyrophosphate, and then mixing the dispersion with urea-ammonium nitrate solution (32 percent N) to yield a product with 30 percent N and 2 percent clay. However, this type of suspension does not contain crystals of urea or ammonium nitrate and is therefore of lower concentration than could be attained if these solids were present.
A urea-ammonium nitrate slurry containing urea solids in saturated urea-ammonium nitrate solution is taught in Brownlie '449, supra. This product was composed of two phases, the solid phase being urea and the liquid phase being a saturated urea-ammonium nitrate solution. In this slurry, the density of the liquid phase was equal to or greater than the density of the solid phase, so the solid phase floated in liquid phase rather than settling out as a sediment.
As determined in the work leading up to that comprising our invention, the urea-ammonium nitrate slurry as described by Brownlie has several undesirable characteristics which make the slurry unusable as a fluid fertilizer for most common applications: (1) Without gelling clay present in the slurry, the urea crystals rapidly grow to large sizes which prevents transfer of the material by pumping, because the long needlelike urea crystals mesh together and clogs pumps and pipelines. Also, such a product cannot be applied to the soil, because the large urea crystals clog spray nozzles used in fluid fertilizer application equipment. (2) Since the density of the liquid phase is equal to or greater than that of the solid phase and since no gelling clay is present in the slurry, the solids float in the liquid phase rather than being homogeneously distributed. Thus, the product contains a slurry layer and a clear liquid layer therebeneath. To transfer the material by pumping, the product must be constantly agitated in order to keep the solid phase homogeneously distributed; otherwise, the clear liquid could be removed separately of the solid phase and prevent removal of the remaining solid phase, because the remaining solid phase does not contain enough liquid to impart fluidity to the solids. Also, if the product is allowed to stand unagitated for any length of time, the solid layer at the top of the mixture becomes hard and encrusted because the urea crystals grow and mesh together in a solid mass, and therefore, it becomes impossible to remove anything but the bottom clear liquid layer. Therefore, the product cannot be stored for any length of time without constant agitation. Even if stored with agitation, the urea crystals grow rapidly and render the product unusable as a fluid fertilizer. It is therefore clear that fluid fertilizers that contain solids which are not homogeneously distributed, whether it be floating or settling solids, are commercially unusable. (3) The products described by Brownlie are unusable in most fluid fertilizer applications because, in addition to the above-described disadvantages, the compositions of the products are such that they cannot be stored, pumped, or applied to the soil at the low temperatures normally encountered during these operations in the fertilizer industry, i.e., 32.degree. F., and below. It is common practice in the fertilizer industry to store liquid fertilizers through the winter and to begin blending operations and application of the fertilizers to the soil in late winter and early spring. Thus, fluid fertilizers must not only be capable of storage for long periods under adverse conditions without loss of quality, but must also remain fluid at low temperatures. The products described by Brownlie solidify at temperatures far above those which fluid fertilizers must often be stored and handled. Because of the excessive time required for thawing, solidification of fluid fertilizers during winter in large storage tanks prevents removal of the fertilizer at the time it is required for blending and/or application to the soil in spring.